cosmetics
avoid possible nullpointer exception
ignore vidada.thumbs folder when scanning libraries

A bit ugly, but the current io.locations file filter system is a bit
hacky when dealing with recursive folder filtering...
fallback to MemoryImageCache when local cache is not avaiable
ICredentialManager as parameter for CacheKeyProvider
upgraded to db4o 8.1

Sadly, even 8.1 seems to fail on Android > 4.0 with enum
NotStoreableException.